@KMMcD, it varies. If you subscribe to anything through Roku the subscriptions are tied to the Roku account, so any devices linked to the same account will automatically have access to the same subscriptions. It's about the only advantage I can think of for subscribing to anything through Roku, but most would still recommend bypassing third parties like Roku, Amazon, Apple, etc. and always subscribe directly with the provider. For those direct subscriptions, some participate in something called "Single sign-on", but I couldn't tell you which ones and I don't know how it works exactly. For other sign-ins where there's not necessarily a subscription involved (YouTube comes to mind), I believe you have to sign in on every device. That's the easiest answer to give - you'll have to sign in on every device, but there are exceptions.
This page has a little more information:
How to add a Roku® streaming device to your Roku account | Official Roku Support
Roku Community Streaming Expert
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